Conventionally, power modules are used for controlling heavy-current for electric vehicles, electric railcars and machine tools. As insulating substrates for such power modules, there is used a metal/ceramic bonding circuit board wherein a metal circuit plate is bonded to the surface of a ceramic substrate.
Recently, there has been a great increase in the heating value of insulating substrates for power modules by the high-output and high-density packing characteristics of electronic parts, such as semiconductor chips, mounted on the substrates, and there has been a great increase in the use of metal/ceramic bonding circuit boards using a ceramic substrate of aluminum nitride sintered body having a high thermal conductivity.
Ceramic substrates used in such metal/ceramic bonding circuit boards are usually produced by sintering a plurality of molded bodies of aluminum nitride powder piled on each other. Therefore, in order to prevent the adhesion of ceramic substrates to each other, a release agent, such as boron nitride (BN) powder, is arranged between the molded bodies. This release agent remains on the surface of the ceramic substrates after sintering. If such ceramic substrates having the release agent remaining on the surface thereof are used, there is a problem in that the bonding strength of the ceramic substrate to the metal circuit plate of the metal/ceramic bonding circuit board is lowered.
In order to solve such a problem, there is proposed a method for bonding a metal plate to a ceramic substrate after a honing treatment for jetting abrasive grains to the surface of the ceramic substrate is carried out (see, e.g., Japanese Patent Laid-Open Nos. 2002-171037 and 2005-98265).
However, in such a method, the repeated generation of heat from electronic parts, such as semiconductor chips, mounted on the metal/ceramic bonding circuit board causes the deterioration of the strength of the ceramic substrate of the metal/ceramic bonding circuit board and/or causes cracks to be easily produced in the ceramic substrate, so that there is a problem in that the heat cycle resistance deteriorates.